1 Answer
1

OWA was written to only decrypt messages on your own machine, not on the mail server. (Mostly due to the obvious – the mail server doesn't have your decryption key.) For this, it needs an ActiveX 'control' to be installed, which only works in Internet Explorer.

If you do not have or don't want to use Internet Explorer, try configuring a desktop email client such as Thunderbird, Windows Live Mail, or MS Office Outlook – all of them support S/MIME. (Most of them require that the server have IMAP enabled, however.)